The present invention relates to a process for ion-nitriding a metal workpiece by means of a glow discharge.
An ion-nitriding process is a process of hardening the surface of a metal workpiece in which the workpiece is subjected to a glow discharge in an atmosphere of a nitrogen-containing gas so that the nitrogen ions produced by the glow discharge are caused to bombard upon the workpiece to effect nitriding. As the nitriding medium, a gas mixture containing nitrogen and hydrogen has been generally employed. In such ion-nitriding process, a workpiece is bombarded by nitrogen ions and hydrogen ions to be heated and caused to release, from the surface thereof, Fe atoms which will react with atomic nitrogen to produce FeN. The FeN compound is deposited on the surface of the workpiece, which has been cleaned by the action of the bombardment of the ions, so as to increase the nitrogen potential. Through such mechanism, the ion-nitriding process is effective to provide a finely nitrided surface layer on a metal workpiece. The ion-nitriding process is also advantageous in that the process is carried out under a vacuum so as to minimize the consumption of the processing gas and further that there is no risk of air pollution.
However, the ion-nitriding process has been conventionally practised in such a manner that the composition of a nitriding gas medium, most generally a gas mixture of nitrogen and hydrogen, is constant throughout the ion-nitriding process, and is disadvantageous in that it takes a long period of time to obtain a desired product having a uniform and sufficiently thick nitrided surface layer. Thus, the conventional ion-nitriding process consumes much energy per cycle of the ion-nitriding treatment; in other words, it does not efficiently use the energy supplied by the glow discharge.
Accordingly, a principal object of the present invention is to overcome the above disadvantages and to provide an improved process for ion-nitriding a metal workpiece, by which the workpiece is ion-nitrided in a very short period of time to produce a product having a uniform and sufficiently thick nitrided surface layer.